For the purpose of preventing corrosion of boilers, a oxygen scavenger is incorporated into a water line to be fed to boilers. Hitherto, hydrazine (N.sub.2 H.sub.4) or sodium sulfite (Na.sub.2 SO.sub.3) has widely been used as such an oxygen scavenger. Of the two, N.sub.2 H.sub.4 may easily be analyzed and can easily be controlled for the concentration, while the safety thereof to human bodies is problematic. Accordingly, use of the compound in hospitals and food factories is refrained from. On the other hand, Na.sub.2 SO.sub.3 is accepted as an additive to foods here in Japan, and it is utilized as a highly safe oxygen scavenger. However, since the reaction of the compound with oxygen is too rapid, the compound reacts with the dissolved oxygen too rapidly in the dissolution tank to cause lowering of the concentration of the compound in the chemical solution. Accordingly, the compound could not sufficiently remove the oxygen from the water as fed into boiler and therefore corrosion of the inside of boiler could not surely be effected.
As other oxygen scavengers which are free from the drawbacks of the said N.sub.2 H.sub.4 and Na.sub.2 SO.sub.3, D-glucose, tannin and lignin have been proposed.
In addition, a boiler compound containing sodium phosphate and sodium hydroxide has also been used for the purpose.
On the other hand, Japanese Patent Application Laid-Open No. 49-30283 has proposed a water-treating agent containing starch phosphate and a water-soluble high polymer substance. Concretely, it illustrates a technique of adding the said water-treating agent to a cooling water line in a determined amount of from 1000 to 2000 ppm as a scale-inhibitor.
With respect to the oxygen scavengers of D-glucose, tannin and lignin, there is unknown a means for detecting the concentration of the said agent in the water fed into a boiler and therefore control of the concentration of the said agent in the water is impossible. Under the situation, where such an oxygen scavenger is used, the dissolved oxygen in the water fed into boiler is analyzed and the amount of the oxygen scavenger to be added is controlled in accordance with the maximum amount of the dissolved oxygen thus analyzed.
However, the amount of the dissolved oxygen in the water fed to boiler noticeably varies every day, depending upon the kind of the feed water to boiler, and such variation is seasonally remarkable. Accordingly, it is extremely difficult to determine the optimum amount of the oxygen scavenger to be added from the result of analysis of the dissolved oxygen. If the amount of the oxygen scavenger added is insufficient, the boiler would be corroded. On the contrary, if it is too much, the excess oxygen scavenger would transfer to the vapor generated so that the vapor would thereby have an peculiar odor. Further, addition of such excess oxygen scavenger would cause elevation of the cost.
As to the liquid boiler compound containing sodium phosphate and sodium hydroxide, since it contains two kinds of sodium salts as the active components in combination, the active components could be blended only in a determined proportion in preparing the compound and the compound could not display a sufficient effect. In addition, the compound has another drawback that the means of handling the compound is complicated.
The water-treating agent containing starch phosphate and a water-soluble high polymer substance, as illustrated in Laid-Open No. 49-30283, is a scale-inhibitor for a cooling water line. Comparing a cooling water line and a boiler water line, the former is operated under far milder conditions for pH and temperature than the latter. Accordingly, the starch phosphate as contained in the agent is not substantially hydrolyzed and the agent does not display an oxygen scavenging effect in the former line. On these grounds, the effect by the hydrolysate from starch phosphate, which will be mentioned hereinafter, could not be obtained in the cooling water line. Laid-Open No. 49-30283 does not suggest the technical idea of removing the dissolved oxygen by the use of the said water-treating agent.